WASHINGTON President Barack Obama warned Syria on Friday that its use of chemical weapons would be a "game changer" for the United States but made clear he was in no rush to intervene in the civil war there on the basis of evidence he said was still preliminary.

Speaking a day after the disclosure of U.S. intelligence that Syria had likely used chemical weapons against its own people, Obama talked tough while calling for patience as he sought to fend off pressure for a swift response against Syrian President Bashar al-Assad.

"Horrific as it is when mortars are being fired on civilians and people are being indiscriminately killed, to use potential weapons of mass destruction on civilian populations crosses another line with respect to international norms and international law," Obama told reporters at the White House as he began talks with Jordan's King Abdullah.

"That is going to be a game changer," he said. But Obama stopped short of declaring that Assad had crossed "a red line" and described the U.S. intelligence evaluations as "a preliminary assessment."

While some more hawkish lawmakers have called for a U.S. military response and for the arming of anti-Assad rebels, several leading congressional voices urged a calmer approach after Secretary of State John Kerry briefed them.

"This is not Libya," said Nancy Pelosi, the senior Democrat in the House of Representatives, referring to the relative ease with which a NATO bombing campaign helped overthrow Muammar Gaddafi in 2011. "The Syrians have anti-aircraft capability that makes going in there much more challenging."

U.S. officials said on Thursday the intelligence community believes with varying degrees of confidence that Assad's forces used the nerve agent sarin on a small scale against rebel fighters.

Obama had warned earlier that deployment of chemical weapons by the Syrian government would trigger unspecified consequences, widely interpreted to include possible U.S. military action.

Aides have insisted that the Democratic president will need all the facts before he deciding on action, making clear it is mindful of the lessons of the start of the Iraq war more than a decade ago.

Then, the Republican administration of President George W. Bush used inaccurate intelligence to justify the invasion of Iraq in pursuit of nuclear, chemical and biological weapons that turned out not to exist.

Syria denies using chemical weapons in the two-year-old conflict in which more than 70,000 people have been killed.

NO 'AIRTIGHT CASE' SO FAR

Ghassan Hitto, the Syrian opposition's elected interim prime minister, said in an interview with CBS television that the opposition needed a no-fly zone, surgical air strikes, and the establishment of safe passages from the U.S. government so aid could be delivered to the Syrian people more effectively.

"We are certain that this regime has used chemical weapons against the Syrian people," he said.

"We are not asking for boots on the ground. We are not asking for any U.S. soldiers or any British soldiers or any foreign soldiers to come in and put their lives at risk."

U.S. officials said the evaluation that Syria probably used chemical weapons was based in part on "physiological" samples but have refused to say exactly where they came from or who supplied the material.

White House spokesman Jay Carney said the evidence so far of Syrian chemical weapons use was not an "airtight case" and declined to set a deadline for corroborating reports.

Obama and his aides also appeared intent on deflecting pressure for swift action by stressing the need for a comprehensive U.N. investigation on the ground in Syria - something Assad has blocked from going forward.

The United States has resisted being dragged militarily into Syria's conflict and is providing only non-lethal aid to rebels trying to overthrow Assad. Washington is worried that weapons supplied to the rebels could end up in the hands of al Qaeda-linked fighters.

But acknowledgment of the U.S. intelligence assessment appeared to move the United States closer - at least rhetorically - to some sort of action in Syria, military or otherwise.

Carney said Obama would consider a range of options, should it be determined that Syria has used chemical weapons.

"It's important to remember that there are options available to a commander in chief in a situation like this that include but are not exclusive to that option," he said.

Some U.S. experts warn that Obama risks further emboldening Assad if he acts too slowly or not all, but the White House must also keep in mind polls showing most Americans, weary of wars in Iraq and Afghanistan, are against new military entanglements.

As a result, Obama is unlikely to turn to military options quickly or without allies joining him.

Those options vary from limited one-off missile strikes, perhaps one of the least complicated scenarios, to more bold operations like carving out no-fly, "safe zones." One grim scenario envisions sending tens of thousands of U.S. forces to help secure the chemical weapons.

LITTLE CHANCE SEEN FOR DIPLOMACY

Current and former U.S. officials see little chance of achieving success through the two main diplomatic options: persuading Russia to increase pressure on Syria at the U.N. Security Council, or pressuring Assad to negotiate his own departure.

"There is no evidence of any interest on the part of Assad, who seems to think Iran and Hezbollah and Russia can pull his chestnuts out of the fire," said Fred Hof, who was a top State Department official working on Syria until September.

The Obama administration's sudden disclosure of its chemical weapons findings came just two days after it played down an Israeli assessment that there had been repeated use of chemical weapons in Syria. France and Britain have also concluded that evidence suggests chemical arms have been used.

It is unclear why the Obama administration changed its mind so quickly this week.

Weapons inspectors will determine whether banned chemical agents were used only if they are able to access sites and take soil, blood, urine or tissue samples and examine them in certified laboratories, according to the Organisation for the Prohibition of Chemical Weapons, which works with the United Nations on inspections.

Assertions of chemical weapon use in Syria by Western and Israeli officials citing photos, sporadic shelling and traces of toxins do not meet the standard of proof needed for a U.N. team of experts waiting to gather their own field evidence, the organization said.

U.S. President Barack Obama (R) and Jordan's King Abdullah await questions from the press prior to a bilateral meeting in the Oval Office of the White House to discuss issues like the Syrian crisis and regional security, in Washington, April 26, 2013.